Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Common stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit	Total
Balance at Dec. 31, 2017	$ 6	$ 472,552	$ (766)	$ (446,375)	$ 25,417
Balance (in shares) at Dec. 31, 2017	54,040,000
Issuance of common stock upon exercise of stock options		13,331			13,331
Issuance of common stock upon exercise of stock options (in shares)	1,967,000
Issuance of common stock under employee stock purchase plan ("ESPP")		3,617			3,617
Issuance of common stock under employee stock purchase plan ("ESPP") (in shares)	391,000
Issuance of common stock upon exercise of warrants		6,762			6,762
Issuance of common stock upon exercise of warrants (in shares)	333,000
Issuance of common stock for offering	$ 1	96,776			96,777
Issuance of common stock for offering (in shares)	5,175,000
Vesting of restricted stock (in shares)	177,000
Stock based compensation		14,198			14,198
Unrealized gain on available-for-sale securities			214		214
Net loss				(128,154)	(128,154)
Balance at Dec. 31, 2018	$ 7	607,236	(552)	(574,529)	32,162
Balance (in shares) at Dec. 31, 2018	62,083,000
Issuance of common stock upon exercise of stock options		13,041			13,041
Issuance of common stock upon exercise of stock options (in shares)	2,464,000
Issuance of common stock under employee stock purchase plan ("ESPP")		4,323			4,323
Issuance of common stock under employee stock purchase plan ("ESPP") (in shares)	268,000
Issuance of common stock for offering	$ 1	323,409			323,410
Issuance of common stock for offering (in shares)	12,624,000
Issuance of common stock to Orbimed		507			507
Issuance of common stock to Orbimed (in shares)	25,000
Vesting of restricted stock (in shares)	541,000
Stock based compensation		28,624			28,624
Unrealized gain on available-for-sale securities			1,471		1,471
Cumulative-effect adjustment upon adoption		(185)		185
Net loss				(124,827)	(124,827)
Balance at Dec. 31, 2019	$ 8	976,955	919	(699,171)	$ 278,711
Balance (in shares) at Dec. 31, 2019	78,005,000				78,005,000
Issuance of common stock upon exercise of stock options		23,524			$ 23,524
Issuance of common stock upon exercise of stock options (in shares)	2,092,000
Issuance of common stock under employee stock purchase plan ("ESPP")		7,114			7,114
Issuance of common stock under employee stock purchase plan ("ESPP") (in shares)	234,000
Issuance of common stock for offering	$ 1	270,649			270,650
Issuance of common stock for offering (in shares)	4,792,000
Vesting of restricted stock (in shares)	1,100,000
Stock based compensation		50,171			50,171
Unrealized gain on available-for-sale securities			3,340		3,340
Cumulative-effect adjustment upon adoption				(404)	(404)
Equity component of Convertible Notes, net		82,873			82,873
Net loss				(229,743)	(229,743)
Balance at Dec. 31, 2020	$ 9	$ 1,411,286	$ 4,259	$ (929,318)	$ 486,236
Balance (in shares) at Dec. 31, 2020	86,223,000				86,223,000